System and method for the mitigation of multipath and the improvement of signal-to-noise ratios in time division multiple access(TDMA) location networks

ABSTRACT

A positioning system that includes a plurality of chronologically synchronized Time Division Multiple Access (TDMA) Positioning-Unit Devices and a position receiver incorporating a TDMA Adaptive Directional Antenna Array is disclosed. The plurality of chronologically synchronized Positioning-Unit Devices, positioned at known locations, transmit positioning signals in a predetermined Time Division Multiple Access (TDMA) sequence, such that each Positioning-Unit Device has a unique transmission time slot. The TDMA Adaptive Directional Antenna Array is configured to consecutively steer a directional receive antenna in spatial synchronization with the plurality of Time Division Multiple Access (TDMA) Positioning-Unit Device transmissions, such that the directional receive antenna is oriented toward the currently transmitting Positioning-Unit Device, or the directional receive antenna is oriented toward the origin of the currently received positioning signal. The TDMA Adaptive Directional Antenna Array is controlled by a deterministic algorithm based on the knowledge of the Positioning-Unit Device locations, TDMA Adaptive Directional Antenna Array location, TDMA Adaptive Directional Antenna Array attitude, network Time Division Multiple Access (TDMA) transmission sequencing, Positioning-Unit Device positioning signal propagation delays, and network time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. § 371 basedupon international application no. PCT/AU03/01246, filed 19 Sep. 2003and published in English on 1 Apr. 2004 under international publicationno. WO 2004/027927, which claims priority to Australian patentapplication no. AU2002951632, filed 20 Sep. 2002.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods forgenerating precise position determinations for a mobile apparatus in thepresence of noise and multipath interference. In particular, the presentinvention relates to the mitigation of code and carrier-phase multipathand the improvement of signal-to-noise ratios in received positioningsignals generated by Time Division Multiple Access (TDMA) locationnetworks.

BACKGROUND OF THE INVENTION

One of the largest error sources in all radio frequency (RF) positioningsystems is multipath. Multipath refers to the phenomenon of a signalreaching a receive antenna via two or more paths. Typically, a receiveantenna receives the direct signal and one or more signals reflectedfrom structures in the receive antenna's vicinity. The subsequent rangemeasurements determined by a position receiver are the sum of thereceived signals, which are generally measured “long” due to the delayednature of the multipath reflections. Therefore, multipath reflectionscause code-based pseudorange biases in location networks which cansubstantially degrade absolute position accuracy measured by a positionreceiver. Furthermore, multipath reflections which arrive at the receiveantenna with phases different to those of the direct signal will sumdestructively with the direct signal, and therefore cause a loss ofreceived signal power, known as signal fading. Moderate signal fadingcauses measured carrier phase errors of up to +/−90 degrees, andpseudorange biases in the tens of metres. Severe signal fading causesreceiver tracking loop destabilization, cycle slips, pseudorange biasesin the hundreds of metres, and possible complete loss of lock on thepositioning signal. Moreover, the unintentional measurement of off-axismultipath signals corrupts receiver Doppler measurements, leading tosignificant degradation in the accuracy of velocity and carrier rangemeasurements in a position receiver. This makes velocity measurementsread “low”, and integrated carrier phase measurements range “short”.

Received signal-to-noise ratios of positioning signals also affect themeasured precision of ranging signals. In general, the greater thereceived signal strength the better accuracy of the measurement.Signal-to-noise ratios are degraded by (1) increasing distance from thetransmission source, (2) signal attenuation caused by line-of-sightobstructions, such as buildings and foliage, (3) multipath signalfading, and (4) an increased noise floor caused by intentional orunintentional signal jammers emitting signals on the positioning signalfrequency.

Prior art methodologies for noise and multipath mitigation using antennadesign have focused on two key areas; (1) multipath limiting antennas,and (2) Programmable Multi-beam Antenna Arrays. Multipath limitingantennas shape the receive antenna gain pattern to reduce the strengthof reflected off-axis signals. The most common form of this antennabeing the so-called choke ring antenna used in GPS applications formitigating satellite signal ground reflections. Multipath limitingantennas traditionally position a directional gain antenna in a fixedorientation, generally positioned facing away from the offendingreflective surface (the ground in the case of the choke ring OPSantenna). This method has limited application in high multipathenvironments, such as indoors or urban areas, where signals reflect frommany directions including buildings, walls, floors, ceilings, furniture,and people.

Programmable Multi-beam Antenna Arrays dynamically shape the receiveantenna gain pattern to reduce the effect of interference sources, suchas intentional signal jammers, and also reduce the affect of multipathsignals. Programmable Multi-beam Antenna Arrays either; (1) combine aplurality of antenna elements to form a gain null in a single antennagain pattern, or (2) combine a plurality of directional gain antennas,each focused on one of the GPS satellites, to form a plurality of peaksin a single antenna gain pattern, or (3) individually monitor aplurality of directional gain antennas, each focused on one of the GPSsatellites, through a matrix of receiver circuitry. A ProgrammableMulti-Beam Antenna Array, which produces a dynamically adjustable gainnull in its antenna gain pattern, has application for mitigating theeffect of signal jamming and thus improving received signal-to-noiseratios by decreasing antenna gain in the direction of the noise source.However, this antenna array has limited application for multipathmitigation in high multipath environments, where multipath signalsreflect from many directions. A Programmable Multi-Beam Antenna Array,which produces a plurality of dynamically adjustable gain peaks in itsantenna gain pattern, has application for mitigating the effect ofsignal jamming and improving received signal-to-noise ratios byincreasing gain in the direction of the satellites and decreasing gainin the direction of the noise source. However, this antenna array haslimited application for multipath mitigation in high multipathenvironments, where a significant amount of multipath is receivedthrough off-axis antenna gain peaks, which are intended for thereception of other satellite positioning signals. Individuallymonitoring a plurality of directional gain antennas through a matrix ofreceiver circuitry has application for mitigating the effect of signaljamming and improving received signal-to-noise ratios, and alsomitigating the affect of multipath. However, a matrix of receivercircuitry has many disadvantages, including: (a) the potential fortime-variant group delay and line biases being introduced intoindividual positioning signal measurements due to the use of disparatereceive paths. These delays change with variations of componenttemperature and supply voltage, thus causing time variant ranging errorsand subsequent position inaccuracies in the position receiver PositionVelocity Time (PVT) solutions; (b) heavy power consumption due to theadditional radio frequency (RF) circuitry, making the position receiverunsuitable for applications where battery weight and size arerestricted; (c) the requirement for proportionally more electroniccomponents than a standard single front-end receiver design, making theposition receiver relatively expensive to produce; and (d) the largeform factor required to house the additional receive circuitry, makingthe receiver larger than a standard single front-end receiver. A systemthat can provide positioning signals free from the encumbrance of severemultipath and degraded signal-to-noise ratios, without any of theseconstraints, is highly desirable. The present invention achieves thisdesirable goal by spatially synchronizing a Time Division MultipleAccess (TDMA) Adaptive Directional

Antenna Array to a chronologically synchronous Time Division MultipleAccess (TDMA) location network, as described below.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a positioning systemand method for making precise code and carrier phase range measurementsfree from the encumbrance of severe multipath, such that accurate codeand carrier phase Position, Velocity, and Time (PVT) solutions may bedetermined.

It is yet a further object of the present invention to provide apositioning system and method for improving measured positioning signalsignal-to-noise ratios (SNR), such that accurate code and carrier phasePosition, Velocity, and Time (PVT) solutions may be determined.

It is yet a further object of the present invention to provide apositioning system and method for improving measured positioning signalsignal-to-noise ratios (SNR) over relatively large distances, or throughradio frequency (RF) obstructed environments, or through radio frequency(RF) jammed environments, such that accurate code and carrier phasePosition, Velocity, and Time (PVT) solutions may be determined.

It is yet a further object of the present invention to provide apositioning system and method for making precise code and carrier phaserange measurements in the presence of noise and multipath utilizing aposition receiver that incorporates a single front-end receiver design.

SUMMARY OF THE INVENTION

The foregoing objects of the present invention are achieved byconsecutively steering a directional receive antenna in spatialsynchronization with a plurality of Time Division Multiple Access (TDMA)Positioning-Unit Device transmissions, such that the directional receiveantenna is oriented toward the currently transmitting Positioning-UnitDevice, or is oriented toward the origin of the currently receivedpositioning signal. The directional receive antenna is controlled by adeterministic algorithm based on the knowledge of the Positioning-UnitDevice locations, directional receive antenna location, directionalreceive antenna attitude, network Time Division Multiple Access (TDMA)transmission sequencing, Positioning-Unit Device positioning signalpropagation delays, and network time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the Time Division MultipleAccess (TDMA) positioning system according to the present invention,wherein a network of chronologically synchronized Positioning-UnitDevices transmit chronologically synchronous Time Division MultipleAccess (TDMA) positioning signals to a roving position receiver, via aspatially unsynchronized TDMA Adaptive Directional Antenna Array. Thespatially unsynchronized TDMA Adaptive Directional Antenna Array isconfigured in an omni-directional gain pattern for positioning signalacquisition.

FIG. 2 is a graphical representation of the Time Division MultipleAccess (TDMA) positioning system according to the present invention,depicting a subsequent time epoch from the time epoch depicted inFIG. 1. A position receiver receives a Time Division Multiple Access(TDMA) positioning signal transmission from a currently transmittingPositioning-Unit Device, via a spatially synchronized TDMA AdaptiveDirectional Antenna Array. The spatially synchronized TDMA AdaptiveDirectional Antenna Array is configured to steer a directional gainpattern toward the currently transmitting Positioning-Unit Device forindividual positioning signal tracking.

FIG. 3 is a graphical representation of the Time Division MultipleAccess (TDMA) positioning system according to the present invention,depicting a subsequent time epoch from the time epoch depicted in FIG.2. A position receiver receives a Time Division Multiple Access (TDMA)positioning signal transmission from a currently transmittingPositioning-Unit Device, via a spatially synchronized TDMA AdaptiveDirectional Antenna Array. The spatially synchronized TDMA AdaptiveDirectional Antenna Array is configured to steer a directional gainpattern toward the currently transmitting Positioning-Unit Device forindividual positioning signal tracking.

FIG. 4 is a graphical representation of a TDMA Adaptive DirectionalAntenna Array that incorporates a plurality of directional gainantennas. Each directional gain antenna is connected to a positionreceiver via a control means that incorporates a plurality of radiofrequency (RF) switches.

FIG. 5 is a graphical representation of the Time Division MultipleAccess (TDMA) positioning system according to the present invention,whereby a position receiver receives Time Division Multiple Access(TDMA) positioning signal via a TDMA Adaptive Directional Antenna Arraythat incorporates a plurality of directional gain antennas.

OVERVIEW

A plurality of chronologically synchronized Positioning-Unit Devices,positioned at known locations, transmit positioning signals in apredetermined Time Division Multiple Access (TDMA) sequence, such thateach transmitter has a unique transmission time slot. A positionreceiver is configured to receive Time

Division Multiple Access (TDMA) positioning signals from the network ofPositioning-Unit Devices via a directionally agile beam antenna Thedirectionally agile beam antenna, known as a TDMA Adaptive DirectionalAntenna Array, is capable of producing a directional gain pattern whichcan be successively steered in a plurality of directions. The TDMAAdaptive Directional Antenna Array is configured with an attitudedetermination means, such as an Inertial Navigation System (INS), toprovide orientation of the TDMA Adaptive Directional Antenna Array.

The position receiver is configured to spatially synchronize the TDMAAdaptive Directional Antenna Array to the Time Division Multiple Access(TDMA) transmission sequence of the network of Positioning-Unit Devicesusing; (1) the Positioning-Unit Device locations determined from thePositioning-Unit Device navigation messages; (2) the Time DivisionMultiple Access (TDMA) transmission sequences determined from thePositioning-Unit Device navigation messages; (3) the TDMA AdaptiveDirectional Antenna Array attitude provided by the attitudedetermination means; (4) the TDMA Adaptive Directional Antenna Arraylocation determined by the position receiver Position Velocity Time(PVT) solution; (5) network time determined by the position receiverPosition Velocity Time (PVT) solution; and (6) positioning signal TimeDivision Multiple Access (TDMA) transmission propagation delaysdetermined from the acquired Positioning-Unit Device locations and thedetermined TDMA Adaptive Directional Antenna Array location. The TDMAAdaptive Directional Antenna Array directional gain pattern issequentially switched to follow the Time Division Multiple Access (TDMA)sequence of the Positioning-Unit Device transmissions, such that thedirectional gain pattern is oriented toward the currently transmittingPositioning-Unit Device, or is oriented toward the origin of thecurrently received positioning signal. As the TDMA Adaptive DirectionalAntenna Array location and attitude change due to user movement theposition receiver adjusts the TDMA Adaptive Directional Antenna Arraydirectional gain pattern azimuth and elevation to follow the currentlytransmitting Positioning-Unit Device, or to follow the origin of thecurrently received positioning signal.

Thus, as detailed below, a TDMA Adaptive Directional Antenna Array is aspecialized receive antenna that is spatially synchronized to achronologically synchronous Time Division Multiple Access (TDMA) networkof Positioning-Unit Devices. The TDMA Adaptive Directional Antenna Arrayprovides both multipath mitigation and improved signal to noise ratiosfor positioning signals received by a position receiver by successivelysteering a directional receive antenna toward the currently transmittingPositioning-Unit Device, or successively steering a directional receiveantenna toward the origin of the currently received positioning signal.

System and Method

Referring to FIG 1., there is depicted a network of chronologicallysynchronized Positioning-Unit Devices 101, 102, 103, & 104, transmittingchronologically synchronous Time Division Multiple Access (TDMA)positioning signals 105, 106, 107, 108. There is also depicted aposition receiver 109, a TDMA Adaptive Directional Antenna Array 110,and an attitude determination means 111. The network of chronologicallysynchronized Positioning-Unit Devices 101, 102, 103, & 104 transmit TimeDivision Multiple Access (TDMA) positioning signals 105, 106, 107, &108, such that each Positioning-Unit Device transmission has its ownunique time slot. The position receiver 109 is configured to receiveTime Division Multiple Access (TDMA) positioning signals 105, 106, 107,108, from the network of Positioning-Unit Devices 101, 102, 103, & 104,through the TDMA Adaptive Directional Antenna Array 110. The TDMAAdaptive Directional Antenna Array 110 incorporates an attitudedetermination means 111, such that the orientation of the TDMA AdaptiveDirectional Antenna Array 110 can be determined. The position receiver109 initially configures the TDMA Adaptive Directional Antenna Array 110in an omni-directional gain pattern 112 to allow acquisition of allPositioning-Unit Devices in-view 101, 102, 103, & 104. The positionreceiver 109 interrogates navigation data transmitted from each acquiredPositioning-Unit Device to determine Positioning-Unit Device location101, 102, 103, & 104, and Positioning-Unit Device Time Division MultipleAccess (TDMA) pulsed transmission sequence 105, 106, 107, & 108. Theposition receiver 109 subsequently performs a Position, Velocity andTime (PVT) solution to determine coarse receiver position, coarsereceiver velocity, and coarse network time. Position, Velocity, and Time(PVT) solutions, also known as “single point position” solutions, arewell known in the art and are not a subject of the present invention.With Positioning-Unit Device locations determined from receivednavigation messages, and coarse receiver location determined by thePosition, Velocity, and Time (PVT) solution the position receiver 109calculates the coarse elevation and azimuth information for allPositioning-Unit Devices in-view 101, 102, 103, & 104. The positionreceiver 109 also determines the TDMA Adaptive Directional Antenna Array110 orientation by processing attitude data 113 provided by the attitudedetermination means 111.

Following on from FIG. 1., and referring now to FIG. 2., there isdepicted a network of chronologically synchronized Positioning-UnitDevices 201, 202, 203, & 204, transmitting chronologically synchronousTime Division Multiple Access (TDMA) positioning signals 205, 206, 207,208, in a subsequent Time Division Multiple Access (TDMA) time slot.There is also depicted a position receiver 209, a TDMA AdaptiveDirectional Antenna Array 210, and an attitude determination means 211.The position receiver 209 interrogates navigation data transmitted fromeach acquired Positioning-Unit Device 201, 202, 203, & 204 to determinePositioning-Unit Device location 201, 202, 203, & 204, andPositioning-Unit Device TDMA pulsed transmission sequence 205, 206, 207,& 208. The position receiver 209 subsequently performs a Position,Velocity and Time (PVT) solution to determine receiver position,receiver velocity, network time, and elevation and azimuth informationfor all Positioning-Unit Devices in-view 201, 202, 203, & 204. Theposition receiver 209 determines the Positioning-Unit Device 201 thatwill commence transmission in the next Time Division Multiple Access(TDMA) time slot by comparing current network time provided by thePosition Velocity Time (PVT) solution, with Positioning-Unit Device TimeDivision Multiple Access (TDMA) transmission sequences provided by thePositioning-Unit Device navigation messages. The position receiver 209determines the direction-of-arrival of the next Positioning-Unit Device201 positioning signal 205 by comparing the calculated azimuth andelevation information derived from the Position, Velocity, and Time(PVT) solution with the current TDMA Adaptive Directional Antenna Array210 attitude provided by the attitude determination means 211. Theposition receiver 209 configures the TDMA Adaptive Directional AntennaArray 210 to produce a directional gain pattern 212, which is steered inthe direction of the next transmitting Positioning-Unit Device 201 atthe commencement of its transmission 205. The position receiver 209continues to direct the TDMA Adaptive Directional Antenna Array 210directional gain pattern 212 toward the origin of the currently receivedPositioning-Unit Device 201 positioning signal 205 until the cessationof its Time Division Multiple Access (TDMA) pulsed transmission 205.

Following on from FIG. 2., and referring now to FIG. 3., there isdepicted a network of chronologically synchronized Positioning-UnitDevices 301, 302, 303, & 304, transmitting chronologically synchronousTime Division Multiple Access (TDMA) positioning signals 305, 306, 307,308, in a subsequent Time Division Multiple Access (TDMA) time slot.There is also depicted a position receiver 309, a TDMA AdaptiveDirectional Antenna Array 310, and an attitude determination means 311.The position receiver 309 interrogates navigation data transmitted fromeach acquired Positioning-Unit Device 301, 302, 303, & 304 to determinePositioning-Unit Device location 301, 302, 303, & 304, andPositioning-Unit Device Time Division Multiple Access (TDMA) pulsedtransmission sequence 305, 306, 307, & 308. The position receiver 309subsequently performs a Position, Velocity and Time (PVT) solution todetermine receiver position, receiver velocity, network time, andelevation and azimuth information for all Positioning-Unit Devicesin-view 301, 302, 303, & 304. The position receiver 309 determines thePositioning-Unit Device 302 that will be received in the next TDMA timeslot by comparing current network time provided by the Position VelocityTime (PVT) solution, with Positioning-Unit Device Time Division MultipleAccess (TDMA) transmission sequences and Positioning-Unit Devicelocation provided by the Positioning-Unit Device navigation messages.The position receiver 309 determines the direction-of-arrival of thenext Positioning-Unit Device 302 positioning signal 306 by comparing thecalculated azimuth and elevation information derived from the Position,Velocity, and Time (PVT) solution with the current TDMA AdaptiveDirectional Antenna Array 310 attitude provided by the attitudedetermination means 311. The position receiver 309 configures the TDMAAdaptive Directional Antenna Array 310 to produce a directional gainpattern 312, which is steered in the direction of the next transmittingPositioning-Unit Device 302 at the commencement of its transmission 306.The position receiver 309 continues to direct the TDMA AdaptiveDirectional Antenna Array 310 directional gain pattern 312 toward theorigin of the currently received Positioning-Unit Device 302 positioningsignal 306 until the cessation of its Time Division Multiple Access(TDMA) pulsed transmission 306. The above described process iscontinuously repeated for all available Time Division Multiple Access(TDMA) time slots.

Accurate position, velocity and time can now be determined by theposition receiver by performing a Position, Velocity and Time (PVT)solution while the TDMA Adaptive Directional Antenna Array is spatiallysynchronized. Off-axis multipath is mitigated through the multipathlimiting effect of the directional gain antenna, and received signal tonoise ratios are increased through the increased forward gain of thedirectional gain antenna. Therefore, more accurate code and carrierphase position solutions can be determined in Time Division MultipleAccess (TDMA) location networks which incorporate a TDMA AdaptiveDirectional Antenna Array than in Time Division Multiple Access (TDMA)location networks that do not.

TDMA Adaptive Directional Antenna Array Methods

A TDMA Adaptive Directional Antenna Array may be created using a varietyof methods. In a first embodiment the TDMA Adaptive Directional AntennaArray incorporates a plurality of spatially distributed antennaelements, with each antenna element incorporating an adjustable phaseand amplitude output. All antenna element outputs are combined to form asingle output, which is fed to the position receiver radio frequency(RF) input. Each antenna element phase and amplitude is controlled via acontrol means, such as a microprocessor, so that various predeterminedphase and amplitude values can be concurrently output from each antennaelement. These outputs, when combined, create various antenna gainpatterns that effectively allow the antenna array to be consecutivelysteered in a plurality of directions. This form of adaptive array isknown as a “phased array” and is well known in the art.

In a second embodiment the TDMA Adaptive Directional Antenna Arrayincorporates a driven antenna element, which is connected to theposition receiver radio frequency (RF) input, surrounded by a pluralityof spatially distributed parasitic antenna elements. A parasitic antennaelement is activated by shorting the antenna element to ground via aradio frequency (RF) switch, which subsequently changes the gain patternof the parasitic array. Each parasitic antenna element RF switch iscontrolled via a control means, such as a microprocessor, so thatvarious combinations of parasitic elements can be activated to createvarious antenna gain patterns, and therefore allow the antenna gainpattern to be consecutively steered in a plurality of directions. Thisform of adaptive array is known as a “Switched Parasitic Antenna Array”and is also well known in the art.

In a third embodiment the TDMA Adaptive Directional Antenna Arrayincorporates a plurality of directional gain antennas, each facing in aunique direction. Each directional gain antenna output is connected to aradio frequency (RF) switch. The outputs of all radio frequency (RF)switches are combined and fed into the position receiver radio frequency(RF) input. Each radio frequency (RF) switch is controlled via a controlmeans, such as a microprocessor, so that each antenna element, or acombination of antenna elements, can be activated at various times tocreate various antenna gain patterns. This allows the TDMA AdaptiveDirectional Antenna Array pattern to be consecutively steered in aplurality of directions. In the preferred embodiment the antenna thatfaces the origin of the currently received Positioning-Unit Devicepositioning signal is activated, with all other antennas deactivated.

Referring now to FIG. 4., there is depicted a TDMA Adaptive DirectionalAntenna Array incorporating a plurality of directional gain antennas410, an attitude determination means 411, a position receiver 409, and acontrol means 412. The TDMA Adaptive Directional Antenna Array 410incorporates eight directional gain antennas 413, 414, 415, 416, 417,418, 419, & 420. Each directional gain antenna 413, 414, 415, 416, 417,418, 419, & 420 has a field-of-view (FOV) of forty five degrees, givinga complete field-of-view (FOV) of 360 degrees. Table 1 shows the angularrange of the field-of-view (FOV) of each directional gain antenna, wherezero degrees is the centre of the field-of-view (FOV) of a referencedirectional gain antenna 413, with angular values increasing in aclockwise direction.

TABLE 1 Directional Gain Antenna FOV Range 413 x ≧ 337.5° or x ≦ 22.5°414  22.5° ≦ x ≦ 67.5° 415  67.5° ≦ x ≦ 112.5° 416 112.5° ≦ x ≦ 157.5°417 157.5° ≦ x ≦ 202.5° 418 202.5° ≦ x ≦ 247.5° 419 247.5° ≦ x ≦ 292.5°420 292.5° ≦ x ≦ 337.5°

The output of each directional gain antenna 413, 414, 415, 416, 417,418, 419, & 420 is connected to individual radio frequency (RF) switches421, 422, 423, 424, 425, 426, 427, & 428, which pass the receivedpositioning signals to the position receiver 409 when activated, ordiscard the received positioning signal to ground when deactivated. Eachradio frequency (RF) switch 421, 422, 423, 424, 425, 426, 427, & 428, isalso connected to a position receiver control means 412, such that theposition receiver 409 can activate each directional gain antenna asrequired and successively steer the TDMA Adaptive Directional AntennaArray gain pattern in a desired direction. An attitude determinationmeans 411 associated with the TDMA Adaptive Directional Antenna Array410 is aligned with the reference directional gain antenna 413. Theattitude determination means 411 provides the orientation of thereference directional gain antenna 413 relative to a common directionalindicator such as true north. This is referred to as the referenceorientation bearing, and is continuously sent by the attitudedetermination means 411 to the position receiver 409.

Referring now to FIG. 5., there is depicted a network of chronologicallysynchronized Positioning-Unit Devices 501, 502, 503, & 504, transmittingchronologically synchronous Time Division Multiple Access (TDMA)positioning signals 505, 506, 507, 508. There is also depicted aposition receiver 509, a TDMA Adaptive Directional Antenna Arrayincorporating a plurality of directional gain antennas 510, an attitudedetermination means 511, and a switching means 512. For illustrativeexample, the network of chronologically synchronized Positioning-UnitDevices 501, 502, 503, & 504 transmit positioning signals 505, 506, 507,508, according to the network Time Division Multiple Access (TDMA)transmission scheme as given in Table 2, though the method of thepresent invention is equally applicable to other Time Division MultipleAccess (TDMA) transmission schemes.

TABLE 2 Time Slot Transmitting Positioning-Unit Device 1 (505) 501 2(506) 502 3 (507) 503 4 (508) 504

The known location of each Positioning-Unit Device 501, 502, 503, & 504,and the network Time Division Multiple Access (TDMA) transmission scheme505, 506, 507, 508, is preferably provided via each Positioning-UnitDevices navigation message, although this information may be provideda-priori to the position receiver 509 by some other means. In thepreferred embodiment the known location of each Positioning Unit Device501, 502, 503, & 504, is provided to the position receiver 509 in EarthCentered Earth Fixed (ECEF) co-ordinates, or some other convenientco-ordinate frame. For this illustrative example latitude and longitudeco-ordinates are described.

Initially, before a Position Velocity Time (PVT) solution is computed bythe position receiver 509, the location of the TDMA Adaptive DirectionalAntenna Array 510 and network time is unknown. Therefore,synchronization of the TDMA Adaptive Directional Antenna Array 510 withthe network TDMA transmission scheme 505, 506, 507, 508, is notpossible. Referring again to FIG. 4, the position receiver 409subsequently activates all RF switches 421, 422, 423, 424, 425, 426,427, & 428, and passes the output from all directional gain antennas414, 415, 416, 417, 418, 419, & 420 during all time slots to thePosition Receiver 409. Thus, positioning signals from the entire 360degree field-of-view (FOV) are acquired. This is analogous to using astandard omni-directional receive antenna. Referring again to FIG. 5,from the coarse Position Velocity Time (PVT) solution calculated usingthe acquired positioning signals 505, 506, 507, & 508 approximate TDMAAdaptive Directional Antenna Array position and network time aredetermined. Given the location co-ordinates of each Positioning-UnitDevice 501, 502, 503, & 504, and the approximate location co-ordinatesof the TDMA Adaptive Directional Antenna Array 510, the positionreceiver 509 can calculate approximate azimuth and elevation angles toeach Positioning-Unit Device from the TDMA Adaptive Directional AntennaArray 510 location. For this illustrative example the Positioning-UnitDevice locations 501, 502, 503, & 504, as shown in Table 3, and thelocation of the TDMA Adaptive Directional Antenna Array 510 are providedin a two-dimensional co-ordinate frame, though the method of the presentinvention is equally applicable to three-dimensional co-ordinate frames.

TABLE 3 Positioning Unit Device Location 501 Latitude: 35.04° Longitude:148.97° 502 Latitude: 35.00° Longitude: 149.10° 503 Latitude: 34.93°Longitude: 149.00° 504 Latitude: 34.99° Longitude: 148.94°

The TDMA Adaptive Directional Antenna Array 510 approximate location isdetermined to be at co-ordinates 35.0° N, 149.0° E. The positionreceiver calculates azimuth to each Positioning-Unit Device 501, 502,503, & 504 from the calculated position of the TDMA Adaptive DirectionalAntenna Array 510, as shown in Table 4 Column 2.

TABLE 4 Positioning-Unit Azimuth Offset Device Azimuth Azimuth Offset[0° ≦ x < 360°] 501 328°    13°  13° 502  90° −225° 135° 503 180° −135°225° 504 258.5°   −56.5°  303.5°  

The attitude determination means 511 determines the referenceorientation bearing of the reference directional gain antenna 513 to be315 degrees. The reference orientation bearing is subtracted from thecalculated Positioning-Unit Device azimuth to form a so-called azimuthoffset for each Positioning-Unit Device, as shown in Table 4, column 3.For example, the first Positioning-Unit Device 501 has an azimuth offsetof 13 degrees. This means the first Positioning-Unit Device is located13 degrees clockwise from the reference directional gain antenna 513 ofthe TDMA Adaptive Directional Antenna Array 510. Likewise, the thirdPositioning-Unit Device 503, with an azimuth offset of −135 degrees islocated 135 degrees anticlockwise from the reference directional gainantenna 513 of the TDMA Adaptive Directional Antenna Array 510.Azimuth-offset values are also mapped into the range [0°≦x<360°], asshown in Table 4 Column 4, by the function:

${F(x)} = \left\{ \begin{matrix}{x,} & {{{{if}\mspace{14mu} x} \geq 0};} \\{{360 + x},} & {{{{if}\mspace{14mu} x} < 0};}\end{matrix} \right.$

The mapped azimuth offsets are then used to select the appropriatedirectional receive antenna for each time slot in the network TimeDivision Multiple Access (TDMA) transmission scheme by using the valuesgiven in Table 1 as a look-up table. The directional gain antenna, whosefield-of-view (FOV) range includes the mapped azimuth offset for a givenPositioning-Unit Device, is activated during the Positioning-UnitDevices Time Division Multiple Access (TDMA) time slot transmission. Forillustrative example, the mapped azimuth offset for the thirdPositioning-Unit Device 503 is 225 degrees, which lies in thefield-of-view (FOV) range of the sixth directional gain antenna 518(202.5°≦x≦247.5°). Thus during the reception of the third Time DivisionMultiple Access (TDMA) time slot positioning signal 507, the controlmeans 512 passes the output of the sixth directional gain antenna 518 tothe Position Receiver 509. Performing this operation for eachPositioning-Unit Device results in the Switching Table shown in Table 5,which indicates the directional gain antenna to be used for each TimeDivision Multiple Access (TDMA) time slot.

TABLE 5 Time Slot Active Directional Gain Antenna Time Slot 1 (505) 513Time Slot 2 (506) 516 Time Slot 3 (507) 518 Time Slot 4 (508) 520

Iterating this process for every update in position and attitude willensure correct alignment of the TDMA Adaptive Directional Antenna Arraywith the network Time Division Multiple Access (TDMA) transmissionscheme.

Time Division Multiple Access (TDMA) Time Slot Overlap

As the distance between a Positioning-Unit Device and a positionreceiver increases the propagation delay of the transmitted positioningsignal increases accordingly. This leads to the possibility that TimeDivision Multiple Access (TDMA) transmissions from a Positioning-UnitDevice may not be received entirely in the Time Division Multiple Access(TDMA) time slot allocated by the position receiver. Consequently, theposition receiver may direct the TDMA Adaptive Directional Antenna Arraydirectional gain pattern away from the origin of the currently receivedPositioning-Unit Device positioning signal at the next allocated timeslot, and miss the tail of the previous Positioning-Unit Devices TimeDivision Multiple Access (TDMA) transmission. The maximum propagationdelay, before a Time Division Multiple Access (TDMA) transmission isreceived entirely in an adjacent time slot, is dependant on thetransmission pulse width used in the network Time Division MultipleAccess (TDMA) transmission scheme. In the preferred embodiment a 50microsecond pulse is transmitted once every millisecond. This provides apropagation distance of 15 kilometres before the transmitted positioningsignal will overlap entirely with an adjacent 50 microsecond time slot.When all Positioning-Unit Devices are in close proximity to the positionreceiver, say less than 1 kilometre, the received positioning signalswill overlap by up to several microseconds. This overlap will cause aminor reduction in received signal-to-noise ratios due to the slightmisalignment of the TDMA Adaptive Directional Antenna Array. When allPositioning-Unit devices are equidistant from the TDMA AdaptiveDirectional Antenna Array, the received Positioning-Unit Devicepositioning signals will not overlap neighbouring Time Division MultipleAccess (TDMA) transmissions. However, if all Positioning-Unit Devicesare equidistant at 15 kilometres from the position receiver, and theposition receiver ignores the approximately 50 microsecond propagationdelay from each Positioning-Unit Device, the TDMA Adaptive DirectionalAntenna Array will be switching one Time Division Multiple Access (TDMA)time slot advanced from the received positioning signals, and thesubsequent Position Velocity Time (PVT) solution may fail. Furthermore,when Positioning-Unit devices distances vary significantly from the TDMAAdaptive Directional Antenna Array, received positioning signals maysignificantly overlap neighbouring Time

Division Multiple Access (TDMA) transmissions. These overlaps can causeconsiderable disruption to the synchronization of the TDMA AdaptiveDirectional Antenna Array if not taken into consideration.

Therefore, the position receiver must take into consideration the signalpropagation delay from each Positioning-Unit Device when calculating theappropriate time to steer the TDMA Adaptive Directional Antenna Arraytoward the origin of the currently received Positioning-Unit Devicepositioning signal. As the position receiver location changes, adeterministic algorithm considers the propagation delay from eachPositioning-Unit Device and adjusts the TDMA Adaptive DirectionalAntenna Array synchronization to best fit the reception time of thePositioning-Unit Device Time Division Multiple Access (TDMA)transmissions. This requires the dynamic adjustment of Time DivisionMultiple Access (TDMA) time slot position and duration for the TDMAAdaptive Directional Antenna Array, depending on position receiverlocation.

Spatial Synchronization without Attitude

In a further embodiment of the present invention a TDMA AdaptiveDirectional Antenna Array may spatially synchronize to a Time DivisionMultiple Access (TDMA) location network without the requirement for anattitude determination means. Attitude determination means are notrequired when; (1) the TDMA Adaptive Directional Antenna Array isstatically positioned with fixed attitude; or (2) the TDMA AdaptiveDirectional Antenna Array is mounted on a user platform which moves withfixed attitude. For illustrative example, a TDMA Adaptive DirectionalAntenna Array may be statically positioned with fixed attitude whenconfigured with a stationary Positioning-Unit Device, which isconfigured to receive positioning signals from other Positioning-UnitDevices in its vicinity. For further illustrative example, a TDMAAdaptive Directional Antenna Array may be statically positioned withfixed attitude when configured with a deformation monitoring positionreceiver. Deformation monitoring position receivers measure the slightmovements of structures, such as bridges and buildings, dependant onsuch variables as temperature and loading.

Furthermore, a TDMA Adaptive Directional Antenna Array does not requireattitude determination means when mounted on a user platform which moveswith fixed attitude. For illustrative example, a crane which moves infixed x, y, and z planes, but exhibits no change in pitch, roll, or yaw,does not require a TDMA Adaptive Directional Antenna Array configuredwith an attitude determination means. Consequently, a position receiveris configured to spatially synchronize a fixed attitude TDMA AdaptiveDirectional Antenna Array to the Time Division Multiple Access (TDMA)transmission sequence of the network of Positioning-Unit Devices using;(1) the Positioning-Unit Device locations determined from thePositioning-Unit Device navigation messages; (2) the Time DivisionMultiple Access (TDMA) transmission sequences determined from thePositioning-Unit Device navigation messages; (3) the TDMA AdaptiveDirectional Antenna Array location determined by the position receiverPosition Velocity Time (PVT) solution; (4) network time determined bythe position receiver Position Velocity Time (PVT) solution; and (5)positioning signal Time Division Multiple Access (TDMA) transmissionpropagation delays determined from the acquired Positioning-Unit Devicelocations and the determined TDMA Adaptive Directional Antenna Arraylocation. The TDMA Adaptive Directional Antenna Array directional gainpattern is sequentially switched to follow the Time Division MultipleAccess (TDMA) sequence of the Positioning-Unit Device transmissions,such that the directional gain pattern is oriented toward the currentlytransmitting Positioning-Unit Device, or is oriented toward the originof the currently received positioning signal. As the TDMA AdaptiveDirectional Antenna Array location changes due to user platform movementthe position receiver adjusts the TDMA Adaptive Directional AntennaArray directional gain pattern azimuth and elevation to follow thecurrently transmitting Positioning-Unit Device, or to follow the originof the currently received positioning signal.

Adaptive Beam-Width

a further embodiment of the present invention the beam-width of the TDMAAdaptive Directional Antenna Array directional gain pattern may bedynamically adjusted depending on position receiver circumstance. Asposition and network time are determined more accurately by the positionreceiver, the azimuth and elevation to each Positioning-Unit Device willalso become better known. Consequently the beam-width of the array maybe narrowed to further mitigate multipath and further improve receivedsignal-to-noise ratios. Beam-widths can be dynamically adjusted in aTDMA Adaptive Directional Antenna Array, in an embodiment whichincorporates a phased array, by increasing the number of spatiallydistributed antenna elements in the array and configuring their phaseand gain outputs to broaden or narrow the resultant beam pattern.Beam-widths can be dynamically adjusted in a TDMA Adaptive DirectionalAntenna Array, in an embodiment which incorporates switched parasiticantenna elements, by increasing the number of parasitic antenna elementsin the array and switching predetermined combinations of these parasiticelements to broaden or narrow the resultant beam pattern. Beam-widthscan be dynamically adjusted in a TDMA Adaptive Directional AntennaArray, in an embodiment which incorporates a plurality of directionalgain antennas, by activating a plurality of adjacent directional gainantennas to broaden the directional gain pattern, or deactivatingadjacent directional gain antennas to narrow the resultant beam pattern.

Attitude Determination Means

A position receiver may determine the attitude, either two-dimensionallyor three-dimensionally, of the TDMA Adaptive Directional Antenna Arrayvia an attitude determination means configured with the TDMA AdaptiveDirectional Antenna Array. The attitude determination means may includean Inertial Navigation System (INS), compass, star tracker, horizonsensor, or other attitude determination sensor. Any attitudedetermination means which provides the TDMA Adaptive Directional AntennaArray with attitude and orientation information may be used to fulfillthe requirements of the present invention. An Inertial Navigation System(INS), as described in the present invention, may include devices suchas an electronic compass, accelerometers and rate gyros. InertialNavigation Systems (INS) and attitude determination systems are wellknown in the art, and are not a subject of the present invention.

Unique Positioning Signals

In the preferred embodiment each Positioning-Unit Device transmits aunique positioning signal, which consists of a carrier component, apseudorandom code component, and a navigation data component. Thecarrier component is a sinusoidal radio frequency wave preferablytransmitted in the 2.4 GHz ISM band, though the method of the presentinvention is equally applicable to other frequency bands. Thepseudorandom number (PRN) code component is modulated upon the carriercomponent, and consists of a unique code sequence which can bedistinguished amongst other pseudorandom code sequences transmitted byother devices on the same carrier frequency. This technique is known asCode Division Multiple Access (CDMA), and is well-known in the art. Thenavigation data component, also referred to as the “navigation message”,is proprietary information modulated upon the pseudorandom codecomponent, and provides a communications link to transfer navigationinformation to Positioning-Unit Devices and roving position receivers.Navigation information may include network time, Positioning-Unit Devicelocations, TDMA transmission sequences, and other desired network data.Each unique positioning signal is pseudo randomly pulsed in apredetermined Time Division Multiple Access (TDMA) transmission scheme,such that each Positioning-Unit Device transmits its unique positioningsignal in a unique time slot.

Time Division Multiple Access (TDMA) Transmissions

In the preferred embodiment each Positioning-Unit Device pulses itstransmission in a pseudorandom Time Division Multiple Access (TDMA)sequence. A 50 microsecond pulse is pseudo randomly transmitted onceevery millisecond, with the pseudorandom sequence repeating every 200milliseconds. This provides a 5% duty cycle with 20 available TimeDivision Multiple Access (TDMA) time slots. The pseudorandom TimeDivision Multiple Access (TDMA) pulse transmission sequence of eachPositioning-Unit Device is transmitted in its navigation message. Aposition receiver determines the pseudorandom Time Division MultipleAccess (TDMA) pulse transmission sequence of each Positioning-UnitDevice by interrogation of each Positioning-Unit Devices navigationmessage. In an alternative embodiment, the pseudorandom Time DivisionMultiple Access (TDMA) pulse transmission sequence may be associatedwith the Positioning-Unit Device Pseudorandom Number (PRN) code. In thisembodiment the position receiver determines Time Division MultipleAccess (TDMA) pulse transmission sequence by associating a receivedPseudorandom Number (PRN) code with a predetermined Time DivisionMultiple Access (TDMA) pulse transmission sequence. A Positioning-UnitDevice may also supply Time Division Multiple Access (TDMA) pulsetransmission sequences, Pseudorandom Number (PRN) codes, and positionco-ordinates for all Positioning-Unit Devices in its vicinity via itsnavigation message, thus allowing a position receiver to quickly acquireand synchronize to neighbouring Positioning-Unit Devices.

It will of course be realized that whilst the above has been given byway of an illustrative example of this invention, all such and othermodifications and variations hereto, as would be apparent to personsskilled in the art, are deemed to fall within the broad scope and ambitof this invention as is herein set forth.

1. A method for determining accurate range measurements in multipath andpoor signal-to-noise ratio environments and subsequently improvinglocation determination at a position receiver incorporating adirectionally agile beam antenna, said position receiver configured toreceive Time Division Multiple Access (TDMA) positioning signalstransmitted by a network of synchronized positioning-unit devices atknown locations, the method comprising: a) calculating the location ofsaid position receiver from said received Time Division Multiple Access(TDMA) positioning signals, and b) steering a directional gain patternof said directionally agile beam antenna exclusively towards the originof the currently received Time Division Multiple Access (TDMA)positioning signal, said steering responsive to: i) said calculatedlocation of said position receiver, and ii) said known locations of saidsynchronized positioning-unit devices.
 2. The method of claim 1, whereinsaid calculating the location of said position receiver form saidreceived Time Division Multiple Access (TDMA) positioning signalsadditionally includes a calculation of a network time of saidpositioning signals transmitted by said positioning unit devices atknown locations, and said steering is additionally responsive to saidcalculated network time.
 3. The method of claim 1, wherein saidcalculating the location of said position receiver from said receivedTime Division Multiple Access (TDMA) positioning signals additionallyincludes the determination of a Time Division Multiple Access (TDMA)sequence of said positioning signals transmitted by saidpositioning-unit devices at known locations, and said steering isadditionally responsive to said determined Time Division Multiple Access(TDMA) sequence.
 4. The method of claim 1, wherein said calculating thelocation of said position receiver from said received Time DivisionMultiple Access (TDMA) positioning signals additionally includes acalculation of the propagation delay of said positioning signalstransmitted by said positioning-unit devices at known locations, andsaid steering is additionally responsive to said calculated propagationdelay.
 5. The method of claim 1 wherein said position receiverincorporating a directionally agile beam antenna is further configuredwith an attitude determination means, said calculating includes anadditional step of determining the attitude of said position receiver,and said steering is additionally responsive to said determinedattitude.
 6. A method for determining accurate range measurements inmultipath and poor signal-to-noise ratio environments in a Time DivisionMultiple Access (TDMA) location network and subsequently improving thelocation determination at a position receiver, the method comprising: a)deploying a plurality of synchronized positioning-unit devices at knownlocations transmitting positioning signals in a Time Division MultipleAccess (TDMA) sequence; b) deploying said position receiver configuredwith a directionally agile beam antenna; c) configuring saiddirectionally agile beam antenna to receive said positioning signalsfrom substantially all directions; d) calculating the location of saidposition receiver from said received positioning signals; e)reconfiguring said directionally agile beam antenna to receive saidpositioning signals from substantially one direction; f) steering adirectional gain pattern of said reconfigured directionally agile beamantenna exclusively towards the origin of the currently receivedpositioning signal, said steering responsive to: i) said calculatedlocation of said position receiver, and ii) said known locations of saidsynchronized positioning-unit devices.
 7. The method of claim 6, whereinsaid calculating the location of said position receiver from saidreceived positioning signals additionally includes a calculation of anetwork time of said positioning signals transmitted by saidpositioning-unit devices at known locations, and said steering isadditionally responsive to said calculated network time.
 8. The methodof claim 6, wherein said calculating the location of said positionreceiver from said received positioning signals additionally includes adetermination of a Time Division Multiple Access (TDMA) sequence of saidpositioning signals transmitted by said positioning-unit devices atknown locations, and said steering is additionally responsive to saiddetermined Time Division Multiple Access (TDMA) sequence.
 9. The methodof claim 6, wherein said calculating the location of said positionreceiver from said received positioning signals additionally includes acalculation of the propagation delay of said positioning signalstransmitted by said positioning-unit devices at known locations, andsaid steering is additionally responsive to said calculated propagationdelay.
 10. The method of claim 6, wherein said position receiverconfigured with a directionally agile beam antenna is further configuredwith an attitude determination means, said calculating includes anadditional step of determining the attitude of said position receiver,and said steering is additionally responsive to said determinedattitude.
 11. A system for determining accurate range measurements inmultipath and poor signal-to-noise ratio environments in a Time DivisionMultiple Access (TDMA) location network, the system comprising: a) aplurality of synchronized positioning-unit devices at known locationstransmitting positioning signals in a Time Division Multiple Access(TDMA) sequence; b) a position receiver configured with a directionallyagile beam antenna; c) means configured to calculate the location ofsaid position receiver from said transmitted positioning signals; d)means configured to steer a directional gain pattern of saiddirectionally agile beam antenna exclusively towards the origin of thecurrently received positioning signal, said steering responsive to: i)said calculated location of said position receiver, and ii) said knownlocations of said synchronized positioning-unit devices.
 12. The systemof claim 11, wherein said means configured to calculate the location ofsaid position receiver additionally includes a means configured tocalculate a network time of said positioning signals transmitted by saidpositioning-unit devices at known locations, and said steering means isadditionally responsive to said calculated network time.
 13. The systemof claim 11, wherein said means configured to calculate the location ofsaid position receiver additionally includes a means configured todetermine a Time Division Multiple Access (TDMA) sequence of saidpositioning signals transmitted by said positioning-unit devices atknown locations, and said steering means is additionally responsive tosaid determined Time Division Multiple Access (TDMA) sequence.
 14. Thesystem of claim 11, wherein said means configured to calculate thelocation of said position receiver additionally includes a meansconfigured to calculate the propagation delay of said positioningsignals transmitted by said positioning-unit devices at known locations,and said steering means is additionally responsive to said calculatedpropagation delay.
 15. The system of claim 11, wherein said positionreceiver configured with a directionally agile beam antenna is furtherconfigured with an attitude determination means, said means configuredto calculate the location of said position receiver includes anadditional means configured to determine the attitude of said positionreceiver, and said steering means is additionally responsive to saiddetermined attitude.
 16. A system for determining accurate rangemeasurements in multipath and poor signal-to-noise ratio environments ina Time Division Multiple Access (TDMA) location network, the systemcomprising: a) a plurality of synchronized positioning-unit devices atknown locations transmitting positioning signals in a Time DivisionMultiple Access (TDMA) sequence; b) a position receiver configured witha directionally agile beam antenna; c) means configured to adjust saiddirectionally agile beam antenna to receive said transmitted positioningsignals from substantially all directions; d) means configured tocalculate the location of said position receiver from said transmittedpositioning signals; e) means configured to readjust said directionallyagile beam antenna to receive said transmitted positioning signals fromsubstantially one direction; f) means configured to steer a directionalgain pattern of said directionally agile beam antenna exclusivelytowards the origin of the currently received positioning signal, saidsteering responsive to: i) said calculated location of said positionreceiver, and ii) said known locations of said synchronizedpositioning-unit devices.
 17. The system of claim 16, wherein said meansconfigured to calculate the location of said position receiveradditionally includes a means configured to calculate a network time ofsaid positioning signals transmitted by said positioning-unit devices atknown locations, and said steering means is additionally responsive tosaid calculated network time.
 18. The system of claim 16, wherein saidmeans configured to calculate the location of said position receiveradditionally includes a means configured to determine a Time DivisionMultiple Access (TDMA) sequence of said positioning signals transmittedby said positioning-unit devices at known locations, and said steeringmeans is additionally responsive to said determined Time DivisionMultiple Access (TDMA) sequence.
 19. The system of claim 16, whereinsaid means configured to calculate the location of said positionreceiver additionally includes a means configured to calculate thepropagation delay of said positioning signals transmitted by saidpositioning-unit devices at known locations, and said steering means isadditionally responsive to said calculated propagation delay.
 20. Thesystem of claim 16, wherein said position receiver configured with adirectionally agile beam antenna is further configured with an attitudedetermination means, said means configured to calculate the location ofsaid position receiver includes an additional means configured todetermine the attitude of said position receiver, and said steeringmeans is additionally responsive to said determined attitude.